Electrochemical liquid treatment equipments

ABSTRACT

The present invention aims to provide electrode compartment structures in electrochemical liquid treatment equipments, which enable stable operation using pure water as an electrode compartment liquid requiring no concentration adjustment without adverse electrode reaction. The present invention relates an electrochemical liquid treatment equipment comprising ion exchange membranes between an anode and a cathode, which has an anode compartment defined by the anode and a cation exchange membrane and a cathode compartment defined by the cathode and an anion exchange membrane ( 12 ), each of the anode compartment and cathode compartment being packed with an ion exchanger ( 14 ) composed of a fibrous a material, each of the anode and cathode ( 11 ) being formed from a liquid- and gas-permeable and electrically conductive material, and the equipment also having an electrode compartment liquid flowing chamber ( 15 ) which is formed behind each of the anode and cathode.

TECHNICAL FIELD

The present invention relates to electrode compartment structures in electrochemical liquid treatment equipments such as electrodialysis, electrolysis and electrodeionization.

BACKGROUND ART

In electrochemical liquid treatment equipments such as electrodialysis, electrolysis and electrodeionization, which are designed for various treatments by passing electricity from electrodes into a liquid to electrolyze or electrically dialyze substance in the liquid, the region containing each electrode plate is often separated by an ion exchange membrane to form an electrode compartment for wetting the electrode in which a liquid from a source other than the treating liquid is circulated. The liquid circulated in this electrode compartment is called electrode compartment liquid and generally consists of a solution containing an electrolyte substance to ensure conductivity. However, the use of the solution containing such an electrolyte substance had the problem that oxidizing or reducing products generated by electrode reactions become to be contained in the electrode compartment liquid to deteriorate the electrode or ion exchange membrane as the operation period progresses. The electrolyte in the electrode compartment liquid must be replenished because the operating voltage of the equipment increases with the resistance in the electrode compartment liquid when the concentration of the electrolyte in the electrode compartment liquid decreases. Moreover, electrode reaction products enter the effluent to degrade the product quality, or the pH of the electrode compartment liquid changes to precipitate substances in the electrode compartment liquid, thereby requiring interventions such as removal of electrode reaction products or replenishment of the electrolyte substance in the electrode compartment liquid.

Thus, the design and operation related to the electrode compartment become complex and expensive materials having high corrosion resistance must be used as electrode materials and ion exchange membranes forming the electrode compartment. When drinking water is to be produced by deionizing brackish water through an electrodialysis (electric dialyzer), for example, an expensive fluorinated ion exchange membrane having high oxidation resistance must be used as an ion exchange membrane forming the anode compartment because chlorine gas is generated at the anode and reacts with water to produce highly oxidizing free chlorine and hypochlorous acid (HOCl).

A method has been proposed by which a bipolar membrane is used in place of ion exchange membranes for diaphragms defining the anode and cathode compartments in an electrodialysis and the electrode compartment liquids for the anode and cathode compartments are separately circulated. If a diaphragm forming an electrode compartment consists of a bipolar membrane, substances in the electrode compartment liquid are scarcely affected because the bipolar membrane is permeable to little electrolyte substance so that only hydrogen ions (cathode compartment) or hydroxide ions (anode compartment) generated in the bipolar membrane flow into the electrode compartment and the hydrogen ions are converted into hydrogen gas at the surface of the cathode and the hydroxide ions are converted into oxygen gas at the surface of the anode. Thus, the method using a bipolar membrane enables continuous electric dialysis without replenishing the anode and cathode compartments with a chemical solution such as an acid or base as an electrolyte during operation. However, this method had the following problems: two electrode compartment liquid flowing systems are required; an expensive bipolar membrane is used; the equipment becomes bulky because the bipolar membrane is difficult to operate at high current densities; and it is difficult to form an anion exchanger region in the bipolar membrane from a fluorinated oxidation-resistant material.

In processes for concentrating and recovering TMAH (tetramethylammonium hydroxide) from waste TMAH solution by electric dialysis, a method for preventing inclusion of impurities into the concentrate has been proposed by supplying a TMAH solution to the anode and cathode compartments to reduce inclusion of impurities into the TMAH solution recovered and also supplying a TMAH solution into an extra liquid chamber formed in the anode compartment in order that heavy amine odor-emitting impurities generated by oxidative decomposition of TMAH in the anode compartment may not penetrate the ion exchange membrane into the concentrate. However, this method had the following disadvantages: the procedure for preparing the TMAH solution supplied to the extra liquid chamber is complex; adverse electrode reactions generating impurities cannot be avoided so that the TMAH solutions circulating in the electrode compartments and extra liquid chamber include impurities and cannot be recovered and recycled.

In electrodeionization apparatus, a method has been proposed by which the anolyte (anode compartment liquid) containing oxidizing substances such as free chlorine discharged from the anode compartment is treated through an activated carbon adsorption column to recycle/reuse the anolyte. However, this method had the following problems: the equipment becomes expensive because of the necessity of the activated carbon adsorption column and its postfilter; and the ion exchange membrane forming the anode compartment cannot be sufficiently prevented from deterioration because oxidizing substances are generated in the anode compartment.

In processes for treating water containing hydrofluoric acid through an electrodeionization (electric deionizer), a method has been proposed by which an electrode compartment liquid channel in each electrode compartment is packed with an ion-conducting spacer in the form of a diagonal net so that the effluent from the electrodeionization poorly containing electrolyte substances can be used as the electrode compartment liquid and adverse electrode reactions are prevented by using the electrode compartment as a deionization compartment to prevent inclusion of electrolyte substances from adjacent compartments. The configuration of an electrode compartment proposed by such a method is shown in FIG. 1. In the conventional electrode compartment structure 1 shown in FIG. 1, an electrode compartment 4 is defined by an electrode 2 and an ion exchange membrane 3, and an ion-conducting spacer 5 having ionic conductivity is packed within the electrode compartment 4 and an electrode compartment liquid inlet 6 and an electrode compartment liquid outlet 7 are connected to the electrode compartment 4. The treated water equivalent to pure water is supplied as an electrode compartment liquid via inlet 6 and discharged via outlet 7 upon ionic conduction by the action of the ion-conducting spacer 5. Such a method of packing the electrode compartment with an ion-conducting spacer is advantageous for avoiding adverse electrode reactions because the treated water equivalent to pure water is supplied to the electrode compartment, but it was insufficient for reducing the operating voltage because a diagonal net having a certain level of mesh size must be used as a spacer to be packed in the electrode compartment in view of the necessity of ensuring a liquid flow in the electrode compartment, which means a small contact area between the electrode surface and ion-conducting spacer and between the ion-conducting spacer and ion exchange membrane and therefore a small area in which ionic conduction occurs. Another problem was that gaseous components generated by electrode reactions were captured by meshes of the spacer and grown to bubbles, which adhered to the electrode surface to increase the voltage drop in the electrode compartment. Thus, a large effluent was needed to remove bubbles from the electrode compartment and a large channel had to be ensured by packing a plurality of expensive ion-conducting spacers to reduce pressure loss, resulting in an increase in cost. Because of these problems, the above method of packing the electrode compartment with an ion-conducting diagonal net spacer involved a high operating voltage and a limited range of usable current density, e.g. this method could be applied to electrodeionization for producing pure water at low operating current densities such as 0.02-0.2 A/dm² but hardly applied to electrodialysis at high operating current densities such as 1-20 A/dm².

As described above, problems remain unsolved in connection with the electrode compartment liquid in the electrode compartments in electrochemical liquid treatment equipments such as electrolysis (electrolyzers), electrodialysis and electrodeionization. In the electrode compartments of common electrodialysis, a water channel is ensured by packing a plastic spacer, but no current flows if pure water is supplied to the electrode compartments because pure water is an insulator. Thus, an electrolyte solution had to be supplied to the electrode compartments in conventional equipments.

If a fluorine ion-containing solution is used, for example, as an electrode compartment liquid, however, the electrode is corroded by hydrofluoric acid with the result that the durability and economy of the electrode are jeopardized and metal ions dissolved from the electrode are diffused into a treated fluid and included as impurities. If a chlorine ion-containing solution is to be used as an electrode compartment liquid, free chlorine is generated at the anode to deteriorate the ion exchange membrane by oxidation and therefore, an expensive fluorinated membrane resistant to oxidative deterioration must be used as an ion exchange membrane forming the anode compartment. If an organic alkali-containing solution is to be used as an electrode compartment liquid, hazardous oxidative decomposition products are generated by electrode reactions as described above.

For these reasons, an inorganic alkaline aqueous solution such as sodium hydroxide, an acid aqueous solution such as sulfuric acid or a salt aqueous solution such as sodium sulfate is commonly used as an electrolyte substance in the electrode compartment liquid, but the concentration of the electrode compartment liquid varies because the electrode compartment functions as a deionization compartment or concentration compartment during operation. Thus, some interventions are needed such as continuous replenishment of the circulating electrode compartment liquid with the electrolyte substance or partial extraction and dilution of the electrode compartment liquid., as well as complex procedures such as the adjustment and control of the concentration of the electrolyte substance in the electrode compartment liquid during operation. Another problem was that if an electrolyte different from the composition to be recovered by the electrochemical liquid treatment equipment is used as an electrode compartment liquid, it was included as impurity in a recycled fluid.

The present invention aims to solve the problems of the prior art as described above and to provide electrode compartment structures in electrochemical liquid treatment equipments, which enable stable operation using pure water as an electrode compartment liquid requiring no concentration adjustment without adverse electrode reaction.

DISCLOSURE OF THE INVENTION

To solve the problems described above, the present invention provides an electrochemical liquid treatment equipment comprising ion exchange membranes between an anode and a cathode, which has an anode compartment defined by the anode and a cation exchange membrane and a cathode compartment defined by the cathode and an anion exchange membrane, each of the anode compartment and cathode compartment being packed with an ion exchanger composed of a fibrous material, each of the anode and cathode being formed from a liquid- and gas-permeable and electrically conductive material, and the equipment also having an electrode compartment liquid flowing chamber which is formed behind each of the anode and cathode.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is a schematic view showing an example of an electrode compartment structure in a conventional electrochemical liquid treatment equipment.

FIG. 2 is a schematic view showing the structure of an embodiment of an electrochemical liquid treatment equipment according to the present invention.

FIG. 3 is a schematic view of a water treatment system used in the examples of the present invention and comparative examples.

EMBODIMENTS OF THE INVENTION

Specific embodiments of electrochemical liquid treatment equipments according to the present invention are explained below with reference to the attached drawings.

FIG. 2 is a schematic view showing an electrode compartment structure in an electrochemical liquid treatment equipment according to an embodiment of the present invention. The electrode compartment structure 10 in the electrochemical liquid treatment equipment A comprises an electrode compartment 14 defined by an electrode 11 and an ion exchange membrane 12; and an electrode compartment liquid flowing chamber 15 behind the electrode 11. The electrochemical liquid treatment equipment A comprises another electrode compartment structure opposed to the electrode compartment structure 10 shown in FIG. 2 (i.e. a reversed counterpart on the right side of the electrode compartment structure 10 in FIG. 2 with the ion exchange membrane 12 being on the left side), and ion exchange membranes are appropriately placed between both electrode compartment structures. In the case where the electrochemical liquid treatment equipment is an electrodeionization, for example, cation exchange membranes and anion exchange membranes are at least partially alternately arranged between the opposed electrode compartment structures to form a deionization compartment and a concentration compartment. In an alternative case where the electrochemical liquid treatment equipment is an electrodialysis for recovering acids and alkalis, cation exchange membranes and anion exchange membranes are at least partially alternately arranged between the opposed electrode compartment structures to form an acid compartment, an ionization compartment, an alkali compartment and a water splitting compartment.

As used herein, the expression “behind the electrode” means “behind” viewed from the electrode on the opposite side and may be translated into “outside” the electrochemical liquid treatment equipment composed of two opposed electrodes.

An electrode compartment liquid inlet 16 and an electrode compartment liquid outlet 17 are connected to the electrode compartment liquid flowing chamber 15. If desired, a vent 18 connected to the electrode compartment liquid outlet 17 may be formed. The electrode compartment 14 is packed with an ion exchanger 13 consisting of a fibrous material. The ion exchanger can be composed of a fibrous material in the form of a woven or nonwoven fabric or the like.

The electrode 11 is formed from a liquid- and gas-permeable and electrically conductive material. Liquid- and gas-permeable and electrically conductive materials that can be used for this purpose include, for example, expanded metals, metal diagonal net materials, grid metal materials, meshed metal materials, foamed metal materials and sintered metal fiber sheets. Specifically, expanded metal materials available from Fukuzawa Wire Net Mfg. under trade name Hi-Expand Metal or foamed metal materials available from Mitsubishi Materials Corporation can be used for the electrode 11.

Feed water is supplied to each compartment of the electrochemical liquid treatment equipment having such an arrangement, and pure water is supplied to the electrode compartment liquid flowing chamber 15 of the electrode compartment structure via the electrode compartment liquid inlet 16. The pure water supplied to the electrode compartment liquid flowing chamber 15 is introduced into the electrode compartment 14 through the water-permeable electrode 11 to impregnate the ion exchanger 13 composed of a fibrous material packed in the electrode compartment 14. The presence of the ion exchanger 13 in the electrode compartment allows good electric conduction using pure water which is an insulator, as an electrode compartment liquid. Moreover, no flow of electrode compartment liquid is required within the electrode compartment 14 itself, so that the electrode compartment can be packed with an ion exchanger composed of a fibrous material in the form of a more dense structure such as a woven or nonwoven fabric as compared with conventional ion-conducting spacers in the form of diagonal nets or the like. Thus, the contact area between the ion exchanger and the electrode surface can be increased as compared with conventional ion-conducting spacers, whereby the electric resistance decreases and the operating voltage can be further reduced. Especially when the current density is high, the problem of heat-induced deterioration of the ion exchanger can be solved because current-induced localized heat generation can be reduced. If an electrode compartment having a conventional structure as shown in FIG. 1 is packed with an ion exchanger composed of such a fibrous material, for example, the flow of the electrode compartment liquid is too hampered to avoid an increase in flow resistance and bubbles generated on the electrode surface described below are confined and remain in the fibrous material to greatly increase the operating voltage.

The electrode compartment structure constructed as above can also eliminate drawbacks due to gases generated by electrode reactions.

On the surfaces of electrodes, electrolysis occurs by electrode reactions during energization. When pure water is flown as an electrode compartment liquid, the following electrolytic reactions of water occur.

The following reactions occur at the anode. 4H₂O→4OH⁻+4H⁺ 4OH⁻→2H₂O+O₂+4e⁻

These reactions are expressed by one formula as follows: 2H₂O→O₂+4H⁺+4e⁻

On the other hand, the following reactions occur at the cathode. 4H₂O→4OH⁻+4H⁺ 4H⁺+4e⁻→2H₂

These reactions are expressed by one formula as follows: 4H₂O+4e⁻→4OH⁻+2H₂

That is, oxygen gas is generated on the surface of the anode simultaneously with hydrogen ions, while hydrogen gas is generated on the surface of the cathode simultaneously with hydroxide ions. In conventional electrode compartment structures as shown in FIG. 1, for example, oxygen and hydrogen gases are thus generated in the electrode compartments to form bubbles, whereby the apparent resistance of the electrode compartment liquid increases to invite an increase in operating voltage.

In the electrode compartment structure according to the present invention, however, the oxygen and hydrogen gases generated on the surfaces of the electrodes are less likely to enter the water-containing ion exchange fibrous material, but easily penetrate the gas-permeable electrode to readily migrate to the electrode compartment liquid flowing chamber 15 behind it and they rise as bubbles 20 through the electrode compartment liquid (pure water) flowing in the chamber. This solves the problem of the increase in operating voltage caused by bubbling in electrode compartments in conventional electrode compartment structures. The bubbles having risen through the electrode compartment liquid is discharged outside the equipment via a vent 18, if it is formed in the electrode compartment liquid outlet 17.

In the present invention, the electrode is required to have the following three functions.

Firstly, it should be made from a corrosion-resistant material that allows good electrode reactions to occur. That is, materials susceptible to oxidative deterioration by energization or electrolysis are less preferred. Secondly, it should be enough strong as a structural component for pressing the ion exchange fibrous material against the ion exchange membrane. If a fibrous carbon material or metal material is used alone for the electrode, for example, it has such low strength that it needs reinforcing materials, which may result in a complex structure. Thirdly, the most important and essential requirement is that the pure water consumed by electrolysis should be supplemented through the electrode from the electrode compartment liquid flowing chamber behind the electrode into the electrode compartment (water-permeability) and that the gases generated in the electrode compartment should be moved through the electrode into the electrode compartment liquid flowing chamber behind the electrode (gas-permeability). Electrode materials satisfying these functions are preferably in the form of expanded metals, metal diagonal net materials, grid metal materials, meshed metal materials, foamed metal materials and sintered metal fiber sheets as mentioned above because of their large pores and satisfactory water-permeability and gas-permeability. In contrast, smooth plate materials having a number of pores such as perforated metals are less preferred because gases generated at the interface between the ion exchange fibrous material and the electrode material are less likely to penetrate the electrode and may increase the electrolytic voltage. Materials such as stainless steel, nickel and platinum-coated titanium are preferably used.

Electrode materials satisfying the above requirements preferably have a pore size of 2 mm or more so that initially generated fine electrolytic bubbles readily penetrate them. Bubbles tend to adhere to pores having a pore size of 1 mm or less, and cannot readily pass through pores having a pore size of 0.5 mm or less. Therefore, the electrode materials preferably have a pore size of 1 mm or more, more preferably 2 mm or more. However, too large pores are not preferred because the contact area with the ion exchange fibrous material decreases and the current density becomes uneven to invite localized ion migration. Therefore, the electrode materials preferably have a pore size of 1 mm-20 mm, more preferably 2 mm-10 mm for practical purposes. The electrode materials preferably have a certain thickness because they are desired to have enough strength for supporting the ion exchange fibrous material without bending to allow the ion exchange fibrous material to come into close contact with the ion exchange membrane, but too large thicknesses are inconvenient for processing and lead to excessively thick electrode compartments. In these respects, electrode materials preferably have a thickness of about 0.6 mm-1.2 mm.

In the electrode compartment structure according to the present invention, the ion exchange fibrous material packed in the electrode compartment has the following three functions. Firstly, it can reduce the resistance to ion migration from the electrode surface to the ion exchange membrane to prevent an increase in operating voltage. Secondly, the entire surface of the fibrous material such as a woven or nonwoven fabric made from fine fibers comes into close contact with the ion exchange membrane to allow ions to penetrate the entire surface of the ion exchange membrane, thereby decreasing the electric resistance of the ion exchange membrane and reducing the energy loss due to heat generation. Thirdly, it serves as cushioning at the interface between the electrode and the ion exchange membrane. Expanded metal materials and metal diagonal net materials have large pores so that when the electrode is directly pressed against the ion exchange membrane, uneven pressure is imposed on the ion exchange membrane to increase the incidence of membrane fracture and ions generated on the electrode surface flow through the interface with the ion exchange membrane, whereby a local current flows through the ion exchange membrane to shorten the life of the ion exchange membrane. According to the present invention, these problems can be solved by packing the electrode compartment defined by an electrode and an ion exchange membrane with an ion exchanger composed of a fibrous material.

Especially preferred ion exchangers in the form of fibrous materials that can be used as packing materials in the electrode compartment in the present invention are those obtained by introducing an ion exchange group onto a polymer fibrous base such as woven or nonwoven fabric by radiation-induced graft polymerization.

Radiation-induced graft polymerization is a technique by which a polymer base is irradiated to form a radical and the radical is reacted with a monomer to introduce the monomer into the base.

The polymer fibrous base that can be used for the purpose of preparing an ion exchanger to be packed in an electrode compartment in the present invention may be either a single fiber formed of a polymer such as a polyolefin polymer, e.g. polyethylene or polypropylene or a composite fiber formed of different core and sheath polymers. Examples of suitable composite fibers include those having a core-sheath structure comprising a sheath formed of a polyolefin polymer such as polyethylene and a core formed of a polymer other than used for the sheath such as polypropylene.

Radiations that can be used in radiation-induced graft polymerization include β-rays, γ-rays, electron beams, etc., among which γ-rays and electron beams are preferably used in the present invention. Radiation-induced graft polymerization includes pre-irradiation graft polymerization involving preliminarily irradiating a graft base and then bringing it into contact with a graft monomer for reaction, and simultaneous irradiation graft polymerization involving simultaneously irradiating a base and a monomer, and either method can be used in the present invention. Radiation-induced graft polymerization includes various manners of contact between a monomer and a base, such as liquid phase graft polymerization performed with a base immersed in a monomer solution; gas phase graft polymerization performed with a base in contact with the vapor of a monomer; or immersion gas phase graft polymerization performed by immersing a base in a monomer solution and then removing it from the monomer solution for reaction in a gas phase; and any method can be used in the present invention.

The ion exchange group to be introduced into the polymer fibrous base for preparing an ion exchanger used as a packing material in the electrode compartment in the present invention is not specifically limited, but various ion exchange groups can be used. For example, suitable cation exchange groups include strongly acidic cation exchange groups such as sulfo group; moderately acidic cation exchange groups such as phosphoric group; and weakly acidic cation exchange groups such as carboxy and phenolic hydroxyl group; and suitable anion exchange groups include weakly basic anion exchange groups such as primary to tertiary amino groups and strongly basic anion exchange groups such as quaternary ammonium groups. Alternatively, ion exchangers having both cation and anion exchange groups as described above can also be used.

These ion exchange groups can be introduced into polymer fibrous bases by graft polymerization, preferably radiation-induced graft polymerization using monomers having these ion exchange groups or using polymerizable monomers having a group capable of being converted into one of these ion exchange groups and then converting said group into the ion exchange group. Monomers having an ion exchange group that can be used for this purpose include acrylic acid (AAc), methacrylic acid, sodium styrenesulfonate (SSS), sodium methallylsulfonate, sodium allylsulfonate, sodium vinylsulfonate, vinyl benzyl trimethyl ammonium chloride (VBTAC), diethyl aminoethyl methacrylate, dimethyl aminopropyl acrylamide, etc. For example, a strongly acidic cation exchange group such as a sulfo group can be directly introduced into a polymer base by radiation-induced graft polymerization using sodium styrene sulfonate as a monomer, or a strongly basic anion exchange group such as a quaternary ammonium group can be directly introduced into a polymer base by radiation-induced graft polymerization using vinyl benzyl trimethyl ammonium chloride as a monomer. Monomers having a group capable of being converted into an ion exchange group include acrylonitrile, acrolein, vinyl pyridine, styrene, chloromethylstyrene, glycidyl methacrylate (GMA), etc. For example, a strongly acidic cation exchange group such as a sulfo group can be introduced into a polymer fibrous base by introducing glycidyl methacrylate by radiation-induced graft polymerization into the base, which is then reacted with a sulfonating agent such as sodium sulfite, or a strongly basic anion exchange group such as a quaternary ammonium group can be introduced into a polymer base by graft-polymerizing chloromethylstyrene to the base, which is then immersed in an aqueous trimethylamine solution to functionalize it with a quaternary ammonium group.

Preferably, at least a sulfo group is introduced if a cation exchange group is to be introduced into a fibrous base or at least a quaternary ammonium group is introduced if an anion exchange group is to be introduced. This is because the operating voltage would increase and it would be difficult to obtain desired performance if existing ion exchange groups were other than sulfo or quaternary ammonium groups that dissociate even in the neutral pH range of pure water used as an electrode compartment liquid. It should be understood that weakly acidic cation exchange groups such as carboxy or weakly basic anion exchange groups such as tertiary amino or lower groups may coexist in the ion exchange fibrous material, but sulfo or quaternary ammonium groups are preferably present in the range of 0.5-3.0 meq/g each expressed as salt splitting capacity. The ion exchange capacity can be increased or decreased by changing the grafting degree, and the ion exchange capacity increases with the grafting degree.

In the electrochemical liquid treatment equipment according to the present invention, it is preferable to pack the anode compartment with a cation exchanger and the cathode compartment with an anion exchanger, respectively. When these ion exchangers are packed in the respective electrode compartments, only a small potential difference is required for ion migration because hydrogen ions H⁺ generated in the anode compartment and hydroxide ions OH⁻ generated in the cathode compartment migrate along the cation exchanger and anion exchanger, respectively. The hydrogen ions having migrated in the anode compartment and hydroxide ions having migrated in the cathode compartment pass through the cation exchange membrane defining the anode compartment and the anion exchange membrane defining the cathode compartment, respectively, to move into the adjacent compartment.

As explained above, pure water can be used as an electrode compartment liquid in the electrode compartment structure according to the present invention because the electrode compartment is packed with an ion exchanger in the form of a fibrous material. In this case, a minimum amount of water to be supplied to the electrode compartment is a makeup feed to the water decomposed by electrode reactions. However, the electrolyte concentration in the electrode compartment liquid gradually increases if only the consumed amount of pure water is made up because even a small amount of electrolyte penetrates the ion exchange membrane by concentration diffusion from the compartment adjacent to the electrode compartment. Thus, it is desirable to prevent an increase in electrolyte concentration by continuously supplying pure water to the electrode compartment. The amount of pure water to be supplied to the electrode compartment can be empirically determined as appropriate by those skilled in the art as the inclusion of electrolyte in the electrode compartment liquid by concentration diffusion varies with the type of electrolyte and other factors.

To save the amount of pure water used, operation can be performed under circulation of the electrode compartment liquid while supplying a given amount of pure water into the electrode compartment liquid and removing the electrolyte through a cartridge ion exchange resin on the circulation path.

In the electrochemical liquid treatment equipment according to the present invention, the thickness of the electrode compartment partially depends on the sizes of other compartments, but typically falls within the range of preferably 2.0-10 mm, more preferably 2.5-3.5 mm. Many experiments using various ion exchange fibrous materials packed in electrode compartments within this range of size showed that the most preferable ion exchange fibrous material to be packed in the electrode compartment for obtaining good and stable effluent quality was a nonwoven fabric base having a thickness of 0.1-1.0 mm, an areal density of 10-100 g/m², a void fraction of 50-98% and a fiber diameter of 10-70 μm.

In the electrochemical liquid treatment equipment according to the present invention, casing materials that can be used to form the electrode compartment and other compartments preferably include, for example, rigid vinyl chloride, polypropylene, polyethylene, EPDM, etc., in view of the availability, easy processing and excellent geometric stability, but are not specifically limited to the list above and any material used in the art for casings of electrodialysis, electrolysis and electrodeionization can be used.

The electrochemical liquid treatment equipment according to the present invention can be specifically in the form of electrodialysis, electrolysis and electrodelonization or the like as explained above. For example, an electrochemical liquid treatment equipment according to the present invention in the form of an electrodeionization can be formed by oppositely arranging two electrode compartment structures according to the present invention shown in FIG. 2 with the ion exchange membranes being inside and at least partially alternately arranging cation exchange membranes and anion exchange membranes between them to form a deionization compartment and a concentration compartment. In this case, the deionization compartment and concentration compartment are preferably packed with ion exchangers in various forms as appropriate as already proposed in the art.

EXAMPLES

The present invention is more specifically explained in detail by way of examples below. The following examples are intended to explain an embodiment of the technical idea of the present invention without limiting the present invention thereto.

Preparation of an Ion Exchange Nonwoven Fabric and an Ion-Conducting Spacer

Table 1 shows the specification of the nonwoven fabric used as a base for preparing an ion exchange nonwoven fabric in the examples. This nonwoven fabric was obtained by thermally bonding a composite fiber comprising a core formed of polypropylene and a sheath formed of polyethylene. TABLE 1 Polypropylene(core)/ Core/sheath composition polyethylene(sheath) Areal Density 50 g/m² Thickness 0.55 mm Fiber diameter 15-40 μm Preparation process of Thermal bonding nonwoven fabric Void fraction 91%

Table 2 shows the specification of the diagonal net used as a base for preparing an ion-conducting spacer in the examples. TABLE 2 Composition Polyethylene Configuration Diagonal net Thickness 0.8 mm Mesh size 6 mm × 3 mm

The nonwoven fabric shown in Table 1 was irradiated with γ-rays in a nitrogen atmosphere and then immersed in a glycidyl methacrylate (GMA) solution and reacted to give a grafting degree of 175%. Then, this grafted nonwoven fabric was sulfonated by immersion in a mixed solution of sodium sulfite/isopropyl alcohol/water. The ion exchange capacity of the resulting ion exchange nonwoven fabric was determined to show that a strongly acidic cation exchange nonwoven fabric having a salt splitting capacity of 2.82 meq/g was obtained.

Separately, the nonwoven fabric irradiated with y-rays as above was immersed in a chloromethylstyrene (CMS) solution and reacted to give a grafting degree of 148%. This grafted nonwoven fabric was functionalized with a quaternary ammonium group by immersing it in a 10% aqueous trimethylamine solution. The resulting ion exchange nonwoven fabric was a strongly basic anion exchange nonwoven fabric having a salt splitting capacity of 2.49 meq/g.

The diagonal net base shown in Table 2 was irradiated with γ-rays in an N₂ atmosphere and then immersed in a mixed solution of glycidyl methacrylate (GMA)/dimethyl formamide (DMF) and reacted to give a grafting degree of 53%. This grafted net was sulfonated by immersion in a mixed solution of sodium sulfite/isopropyl alcohol/water to give a strongly acidic cation-conducting spacer having a salt splitting capacity of 0.62 meq/g.

The diagonal net base shown in Table 2 was irradiated in the same manner as described above and then immersed in a mixed solution of vinyl benzyl trimethyl ammonium chloride (VBTAC)/dimethylacrylamide (DMAA)/water and reacted to give a grafting degree of 36%. This spacer was a strongly acidic anion-conducting spacer having a salt splitting capacity of 0.44 meq/g.

Example 1

An electrodeionization was formed using the ion exchange nonwoven fabrics and ion-conducting spacers obtained as above as well as commercially available ion exchange membranes. A cation exchange membrane from Tokuyama Corp. (trade name: C66-10F) and an anion exchange membrane from Tokuyama Corp. (trade name: AMH) were used to form an electrodeionization having eleven deionization compartments in parallel. Each deionization compartment was packed with the cation exchange nonwoven fabric and anion exchange nonwoven fabric obtained as above to border on the cation exchange membrane and the anion exchange membrane, respectively, as well as a piece of the cation-conducting spacer obtained as above on the side of the cation exchange nonwoven fabric and a piece of the anion-conducting spacer on the side of the anion exchange nonwoven fabric, respectively. Each concentration compartment was packed with a piece of the untreated and non-ionic conductive polyethylene diagonal net. Electrode compartments having the structure shown in FIG. 2 were used in combination with electrodes made from an expanded metal material from Fukuzawa Wire Net Mfg. (trade name: Hi-Expand Metal; mesh size 4.0×8.0 mm, thickness 0.8 mm), and an anode compartment defined by an anode and a cation exchange membrane was packed with a piece of the cation exchange nonwoven fabric obtained as above, and a cathode compartment defined by a cathode and an anion exchange membrane was packed with a piece of the anion exchange nonwoven fabric obtained as above, respectively. Each compartment had a size of 400 mm×600 mm.

The electrodeionization having this structure was used to construct a water treatment system shown in FIG. 3. Wastewater (recycled raw water) 51 from a wet washing process was stored in a raw water tank 52 and fed through an influent line 61 and then sent through an activated carbon cartridge 58 and a filter 59 by the action of a feed pump 55 to the deionization compartment in the electrodeionization 54.

Feed water to the concentration compartment in the electrodeionization 54 was the recycled raw water sent by a feed pump 56 from the raw water tank 52 to a concentrate circulating tank 53. The recycled raw water stored in the concentrate tank 53 was supplied to the concentration compartment in the electrodeionization 54 through a concentrate line 67 by the action of a feed pump 57. The effluent (concentrate) from the concentration compartment was circulated to the concentrate circulating tank 53 through a concentration compartment discharge line 68. A conductivity meter 60 was placed on the concentration compartment discharge line 68 to measure the conductivity of the concentrate, whereby the ionic concentration of the concentrate was monitored. Once the ionic concentration of the concentrate exceeded a level of 2-4 mS/m, a valve 72 was opened to discharge water through a concentrate discharge line 69 into a drainage conduit 70. The circulation loss due to this drainage was made up by supplying raw water 51 to the concentrate circulating tank 53 by the action of the feed pump 56.

Both electrode compartments in the electrodeionization 54 were supplied with a part of effluent (deionized water) from the deionization compartment by branching a deionization compartment outlet line 63 into an electrode feed line 64 and connecting it to both electrode compartments. The effluent from each electrode compartment was returned to the raw water tank 52 via an anode compartment outlet line 65 and a cathode compartment outlet line 66.

When the equipment described above was used and fed with a solution having a hydrofluoric acid concentration of 0.7-0.8 mg/L at a flow rate of 1 m³/hr under a constant current operation (0.05 A/dm²) for 1000 hours, a specific resistance of 16 MO·cm or more was stably ensured in the treated water (effluent from the deionization compartment outlet) 63. Voltage drop in each electrode compartment was determined by measuring the voltage with a platinum wire inserted between the electrode surface and the ion exchange nonwoven fabric and between the ion exchange nonwoven fabric and the ion exchange membrane. The voltage drops in both electrode compartments were stable at 0.6 V in the anode compartment and 0.9 V in the cathode compartment.

After water feeding, the corrosion of the electrodes and the deterioration of the ion exchange membranes and ion exchangers were assessed to show no practical problem.

Comparative Example 1

An electrodeionization was formed in the same manner as in Example 1 except that the electrode compartments in the electrodeionization had a conventional structure as shown in FIG. 1 and the anode compartment was packed with the cation-conducting spacer obtained as above and the cathode compartment was packed with the anion-conducting spacer obtained as above, and this deionizer was used to construct a water treatment system shown in FIG. 3.

When this system was used and fed with a solution having a hydrofluoric acid concentration of 0.7-0.8 mg/L at a flow rate of 1 m³/hr under a constant current operation (0.05 A/dm²) for 1000 hours, a specific resistance of 15 MO·cm or more was stably ensured in the treated water (effluent from the deionization compartment outlet) 63. Voltage drops in the electrode compartments were about 2.4 V in the anode compartment and about 2.8 V in the cathode compartment with a variation of about ±0.2 V during operation.

After water feeding, the corrosion of the electrodes and the deterioration of the ion exchange membranes and ion exchangers were assessed to show no abnormality in the same manner as in Example 1.

Example 2

When the same equipment as used in Example 1 was fed with a solution having a hydrofluoric acid concentration of about 100 mg/L at a flow rate of 1 m³/hr under a constant current operation (2.5 A/dm²) for 200 hours, a specific resistance of 2 MO·cm or more was stably ensured in the treated water (effluent from the deionization compartment outlet) 63. Voltage drop in each electrode compartment was determined by measuring the voltage with a platinum wire inserted between the electrode surface and the ion exchange nonwoven fabric and between the ion exchange nonwoven fabric and the ion exchange membrane. The voltage drops in both electrode compartments were stable at 2.8 V in the anode compartment and 4.7 V in the cathode compartment.

Comparative Example 2

When the same equipment as used in Comparative example 1 was fed with a solution having a hydrofluoric acid concentration of about 100 mg/L at a flow rate of 1 m³/hr under a constant current operation (2.5 A/dm²) in the same manner as in Example 2 for 50 hours, a specific resistance of 1.5-2 MO·cm was obtained in the treated water (effluent from the deionization compartment outlet) 63. Voltage drops in the electrode compartments gradually increased with the operation period from about 7 V in the anode compartment and about 12 V in the cathode compartment at the start of the operation to 13 V in the anode compartment and about 25 V in the cathode compartment after 50 hours. After water feeding, the deterioration of the ion-conducting spacers was assessed to show deterioration as browning at the interface between the ion-conducting spacer and the electrode surface and the interface between the layered ion-conducting spacers. This may be caused by the heat generation induced by a localized large current flow. The electrodes and ion exchange membranes showed no abnormality.

ADVANTAGES OF THE INVENTION

Electrochemical liquid treatment equipments having electrode compartment structures according to the present invention enable water treatment at a stable operating voltage without any drawback caused by conventional electrode compartment structures. 

1. An electrochemical liquid treatment equipment comprising ion exchange membranes between an anode and a cathode, which has an anode compartment defined by the anode and a cation exchange membrane and a cathode compartment defined by the cathode and an anion exchange membrane, each of the anode compartment and cathode compartment being packed with an ion exchanger composed of a fibrous material, each of the anode and cathode being formed from a liquid- and gas-permeable and electrically conductive material, and the equipment also having an electrode compartment liquid flowing chamber which is formed behind each of the anode and cathode.
 2. The electrochemical liquid treatment equipment of claim 1 wherein the liquid- and gas-permeable and electrically conductive material is selected from expanded metals, metal diagonal nets, grid metal materials, meshed metal materials, foamed metal materials and sintered metal fiber sheets.
 3. The electrochemical liquid treatment equipment of claim 1 wherein the ion exchanger composed of a fibrous material is an ion exchange woven or nonwoven fabric prepared by radiation-induced graft polymerization.
 4. The electrochemical liquid treatment equipment of claim 1 wherein the anode compartment is packed with a cation exchanger and the cathode compartment is packed with an anion exchanger.
 5. The electrochemical liquid treatment equipment of claim 1 wherein at least one of the anode compartment and cathode compartment is supplied with pure water or ultrapure water as an electrode compartment liquid.
 6. The electrochemical liquid treatment equipment of claim 1 wherein the electrode compartment liquid flowing chamber has a vent.
 7. The electrochemical liquid treatment equipment of claim 1, which is an electrodeionization comprising cation exchange membranes and anion exchange membranes at least partially alternately arranged between an anode and a cathode to form a deionization compartment and a concentration compartment.
 8. The electrochemical liquid treatment equipment of claim 1, which is an electrodialysis comprising cation exchange membranes and anion exchange membranes at least partially alternately arranged between an anode and a cathode to form an acid compartment, an ionization compartment, an alkali compartment and a water splitting compartment.
 9. The electrochemical liquid treatment equipment of claim 2 wherein the ion exchanger composed of a fibrous material is an ion exchange woven or nonwoven fabric prepared by radiation-induced graft polymerization.
 10. The electrochemical liquid treatment equipment of claim 2 wherein the anode compartment is packed with a cation exchanger and the cathode compartment is packed with an anion exchanger.
 11. The electrochemical liquid treatment equipment of claim 3 wherein the anode compartment is packed with a cation exchanger and the cathode compartment is packed with an anion exchanger.
 12. The electrochemical liquid treatment equipment of claim 9 wherein the anode compartment is packed with a cation exchanger and the cathode compartment is packed with an anion exchanger.
 13. The electrochemical liquid treatment equipment of claim 2 wherein at least one of the anode compartment and cathode compartment is supplied with pure water or ultrapure water as an electrode compartment liquid.
 14. The electrochemical liquid treatment equipment of claim 3 wherein at least one of the anode compartment and cathode compartment is supplied with pure water or ultrapure water as an electrode compartment liquid.
 15. The electrochemical liquid treatment equipment of claim 9 wherein at least one of the anode compartment and cathode compartment is supplied with pure water or ultrapure water as an electrode compartment liquid.
 16. The electrochemical liquid treatment equipment of claim 4 wherein at least one of the anode compartment and cathode compartment is supplied with pure water or ultrapure water as an electrode compartment liquid.
 17. The electrochemical liquid treatment equipment of claim 10 wherein at least one of the anode compartment and cathode compartment is supplied with pure water or ultrapure water as an electrode compartment liquid.
 18. The electrochemical liquid treatment equipment of claim 11 wherein at least one of the anode compartment and cathode compartment is supplied with pure water or ultrapure water as an electrode compartment liquid.
 19. The electrochemical liquid treatment equipment of claim 12 wherein at least one of the anode compartment and cathode compartment is supplied with pure water or ultrapure water as an electrode compartment liquid.
 20. The electrochemical liquid treatment equipment of claim 2 wherein the electrode compartment liquid flowing chamber has a vent. 